METHODS OP MANUFACTURE. 211
SEC. Xllb.—The pig-and-ore lasic process.—The question of working a large proportion of pig-iron is one which all large works are driven to face. In an ordinary stationary furnace the use of an entire charge of pig-iron is objectionable on account of excessive frothing of metal and slag. From the time that the metal is thoroughly melted, when it may contain about 3 per cent, of carbon, until the proportion is reduced to about 1| per cent., the bath resembles soda water more than pig-iron, and it tries to flow out of the doors and to occupy about twice the room it should. In Steelton we have solved the difficulty caused by this frothing by using the tilting furnace rotating about a central axis. (See Chapter VII.) The pig-iron is brought in a melted state from the blast furnace and poured into the open-hearth furnace, a sufficient quantity of iron ore and lime being added. During the combustion of silicon no violent reaction occurs, but immediately afterward a general movement takes place, whereupon the furnace is tipped over until the metal is thrown away from the doors and up on the back side. In this way the capacity of the furnace is practically doubled, while the flame enters and goes out as usual. The furnace is kept in this position for two or three hours, until the bath has quieted down. Meanwhile the slag is trying to froth out of the ends of the furnace and down the ports, but to do so it must flow over the open joint between the port and the furnace. This joint is not wide, but special provision is made to allow the slag to run out through a small hole and fall down beneath the end of the furnace in a slag pit. In this way a considerable quantity is removed and the time of operation lessened.
At some works the slag is removed by a small tap-hole or through the regular door, but under these circumstances the stream continually chills and must be carefully tended. In the arrangement above described there is little tendency to chill, for the flame is constantly playing back and forth through the ports and the slag opening is in the immediate course of the hottest flame. This practice of using direct metal has been in more or less continuous use , for several years on furnaces of fifty tons capacity. Working in this way the iron of the ore is reduced in such quantity that the product of steel, counting both ingots and scrap, exceeds the weight of pig-iron charged by from 4 to 6 per cent, when the charge is entirely pig-iron.